Cardiac arrhythmias are a significant health problem, and atrial fibrillation is a common cardiac arrhythmia. Although atrial arrhythmias may not be as fatal as frequently as ventricular arrhythmias, atrial arrhythmias increase risk factors for other conditions such as embolisms. Further, atrial arrhythmias can contribute to the onset of ventricular arrhythmia.
It is believed that cardiac electrical impulses normally start in a sinoatrial (SA) node, spread through the atria, and progress through the atrial-ventricular (AV) node to the ventricles to complete a heartbeat. Atrial fibrillation is an irregular heart rhythm that originates in abnormal cells in the atria, the upper two chambers of the heart. Muscle fibers in the pulmonary veins, in particular, can be sources of disruptive re-entrant electrical impulses.
One known method of treating atrial fibrillation is by use of medication that is intended to maintain a normal sinus rate and/or decrease ventricular response rates. It is also known to use implant devices such as atrial pacemakers to treat these conditions. Other known methods and devices have been developed for creating therapeutic lesions, e.g., by open-heart or minimally-invasive surgical methods, in the myocardial tissue to block sources of unwanted electrical impulses that are believed to be the source of atrial fibrillation. In this context, ablation has come to mean the deactivation, or removal of function, rather than the removal of the tissue, per se. A number of energy sources may be used for creating these “blocking” lesions that may be transmural and extend across the entire heart wall to isolate the unwanted sources of activation from the rest of the excitable tissue in the heart.
Formation of lesions may be performed using both endocardial and epicardial devices and procedures. Endocardial procedures are performed from within the heart. Since the endocardium primarily controls myocardial functions, there are inherent advantages to generating lesions by applying an energy source to endocardial surfaces. One known manner of applying energy for this purpose is to utilize radio frequency (RF) catheters, which ablate tissue by heating it over about 50° C. Other devices and procedures involve cryo-ablation. Cryo-ablation devices ablate tissue by freezing the tissue to permanently destroy its function. Examples of known lesion formation devices, including cryogenic balloon catheters for use in endocardial ablation and their operation are described in U.S. Patent Application Publication No. 20060084962, U.S. Pat. Nos. 6,027,499; 6,468,297; 7,025,762; 7,081,112 and 7,150,745 and Williams, et al, “Alternative Energy Sources for Surgical Atrial Ablation”, J. Card. Surgery, 2004; 19:201-206, the contents of which are incorporated herein by reference as though set forth in full.
The effectiveness of cryogenic balloon catheters depends on various factors including, for example, successfully delivering high quality cryogenic coolant or refrigerant (generally referred to as “coolant”) from a pump, pressure reservoir or other refrigerant source and to the target site or tissue to be treated. Certain known cryogenic balloon catheters operate as a closed-loop fluid system. Coolant is fed to the catheter at a high pressure, and cryogenic cooling results from evaporation of the coolant resulting from a pressure drop as the cryogenic fluid is sprayed into the interior of a balloon at the catheter tip. The quality of the coolant, e.g., the relative proportion of the coolant, which may be in liquid state, is determined by the local pressure and temperature of the coolant relative to the vapor saturation line for the coolant. For example, 100% saturation may be preferred.
Ideally, the coolant is delivered to the catheter tip at a sufficiently low temperature and a sufficiently high pressure such that the combination of the low temperature and high pressure is above the vapor saturation line for the coolant. However, during use, the pressure of the liquid coolant drops and the temperature of the coolant may increase as the liquid flows through a coolant supply line or hose and the catheter. The resulting pressure drops and higher temperatures negatively impact the quality of the cryogenic coolant that is delivered to the tip of the cryogenic catheter, increasing fluid resistance, which reduces the rate at which liquid coolant is provided to the catheter tip. This diminishes the cryogenic effect of the coolant and the quality of lesions that are formed thereby. For example, if the temperature of liquid nitrous oxide is increased by a certain degree, gas bubbles will form within the nitrous oxide coolant. These bubbles increase fluid resistance of the coolant as the coolant flows through a small diameter supply tube or conduit, thus reducing the rate at which liquid nitrous oxide coolant can be provided to the tip of the catheter to perform cryo-ablation.
One attempt to address these issues is to maintain the liquid coolant in the supply path adequately above the saturation line by increasing the local pressure, lowering the local temperature, or both. For this purpose, it is known to utilize a cryogenic supply console, which is typically located near the clinician and is used to chill the coolant that is supplied to the catheter. These supply consoles are typically large components and incorporate a compressor or a heat exchanger/chiller to provide coolant at desired pressures and temperatures. For example, one known supply console has a large mechanical compressor that is used to liquefy gas refrigerant at a high pressure, and another known supply console has a heat exchanger/chiller to liquefy the refrigerant vapor and deliver it at a low temperature. Such large supply consoles may initially provide coolant having desired characteristics, but the required console equipment is bulky and of such a size that it is not desirable to have them in operating environments. Other difficulties arise from placing such large consoles at a distance from operating environments due to associated warming of the liquid coolant, increased fluid resistance, and decreased coolant flow to the tip of the catheter.